1. Field of the Invention
The present invention relates to a thermally developable photosensitive material, and more particularly relates to a thermally developable photosensitive material containing a compound which protects it from deterioration on storage prior to exposure.
2. Description of the Prior Art
Photographic methods employing a silver halide have been widely employed because the photographic properties such as sensitivity or gradation are better than those of electrophotographic methods and diazo photographic method. However, a silver halide photosensitive material employed in photographic methods must be developed, after exposure, with a developer, followed by stopping, fixing, washing and stabilizing so that the developed images do not change in color or do not fade under light, and so that the undeveloped portions thereof (hereinafter, referred to as "background") do not blacken. Accordingly, these processings are laborious and time consuming, and the handling of chemicals is often hazardous and the operators' hands and clothes are stained. It has been, therefore, desired in photographic methods employing silver halide that dry processings can be carried out without the necessity for processing in solution yet to maintain the processed images stable.
Various methods have been proposed to accomplish this. One method is a thermally developable photosensitive material disclosed in U.S. Pat. Nos. 3,152,904, 3,457,075, 3,635,719, 3,645,739 and 3,756,829, and Canadian Pat. No. 811,677.
In these materials, a silver salt of a long-chain aliphatic carboxylic acid, for example, silver behenate, or silver saccharin, silver benzotriazole, etc. as a light-sensitive element, and a catalytic amount of silver halide are used.
The present invention relates to these types of thermally developable photosensitive materials.
If conventional thermally developable photosensitive materials are stored in the dark, they are desensitized or fogged. The term "fog" is used to describe the phenomenon in which the unexposed portions of the thermally developable photosensitive materials are blackened by thermal development; The tendency toward fog formation upon subsequent development increases after storage of conventional thermally developable photosensitive materials since fogging nuclei are formed. That is, their shelf life is insufficient. As used in this specification, the term "good shelf life" means that the photographic properties of photosensitive materials in storage are the same as or similar to the photographic properties obtained directly after the preparation of the photosensitive materials.
As disclosed in Japanese Patent Application Laid open to Public Inspection No. 125016/1974 and 29463/74, corresponding to U.S. Application No. 558,607 filed Mar. 14, 1975, it has been recognized that acids prevent the generation of fogging nuclei during storage which results in an increase of fog when the thermally developable material is thermally developed. While in regard to the phenomenon observed, when the thermally developable material is thermal developed after long storage, that the image density does not increase and the sensitivity of the material is gradually reduced, it has been assumed that the oxidation of the reducing agent by oxygen in the air during storage causes this phenomenon.
In order to improve the shelf life, various methods have been proposed. One method involves preventing the reducing agent from being oxidized by oxygen in the atmosphere by incorporating an oxidizing agent in the photosensitive or layer or by providing a polymer layer on the photosensitive layer. Another method involves the replacement of phenols or naphthols as a reducing agent by bisphenols as disclosed in Japanese Patent Application No. 81,625/73, corresponding to U.S. Application No. 489,327, filed July 17, 1974, now abandoned or by bisnaphthols as disclosed in Japanese Patent Application Laid Open to Public Inspection No. 6,074/71. However, no method by which the shelf life is remarkably improved has been developed.